As the title suggests. How would one go about this? I tried get alt, surface layer with terrain as child with maximum alt, and with a surface layer as a mask with maximum alt and solid white colour. None of these even effect the terrain or cut if off at the desired altitude. I even tried offsetting by "crunching" the tops with a minimum alt but wasn't getting right results.

Been trying to make plateau type scenery using just PFs as the shapes provided are often very nice compared to warping/fractalizing a SSS imo, but can't grasp how to get my tops right, being mainly just flat, but some curve to the ledges.